Former commissioner John O'Brien and deputies Elizabeth Tavares and William Burke face charges of rigging the agency's hiring apparatus to favor job-seekers who were recommended by state legislators and other officials. Prosecutors say the politically connected applicants were often hired over those who were more qualified.

The three have pleaded not guilty to charges including racketeering, mail fraud and bribery.

U.S. District Court Judge William Young has said it could take up to three days to impanel a jury.

A motion filed by prosecutors late Sunday sought permission to conduct criminal background checks on potential jurors who move beyond the initial screening process. The government said the checks would be important to assure a fair trial and prevent issues from cropping up on appeal.

The request referenced the case of Gary Sampson, who was sentenced to death by a federal jury for carjacking and killing two Massachusetts men during a weeklong crime spree. A judge later ordered a new sentencing trial after finding that a juror had lied about her background.

A proposed questionnaire for potential jurors in the probation trial would ask, among other things, their familiarity with a number of high-ranking judges and lawmakers who could be called to testify.

A revised list of possible defense witnesses submitted Friday included 47 past or present judges, 38 current or former legislators and 18 past or present law enforcement officials.

Young on Monday ruled on several other motions made by attorneys on both sides.

In one, the judge denied a bid by prosecutors to show jurors records from the office of Senate President Therese Murray. The documents, according to the motion, were kept by an aide to Murray and listed people who contacted the office about jobs, including within the probation department.

Young allowed a motion filed by Burke's attorney to exclude from the trial any mention by prosecutors of a 2010 report by independent counsel Paul Ware. He was appointed by Massachusetts court officials to investigate probation department hiring.